This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled Wavelength Division Multiplexed Transmission System And Transmitting Method Using The Same earlier filed in the Korean Industrial Property Office on Sep. 26, 1997, and there duly assigned Ser. No. 97-49091 by that Office.
1. Field of the Invention
The present invention relates to a wavelength division multiplexed transmission system and a transmitting method using the same, and more particularly, to a wavelength division multiplexed transmission system in which amplification is performed after amplification gain is controlled and a transmitting method using the same.
2. Description of the Related Art
With development of the erbium doped fiber amplifier since the early 1990""s which is an optical amplifier, there has been remarkable development in the field of optical transmission. The erbium doped fiber amplifier periodically amplifies a weakened optical signal in order to complement the attenuation of an optical signal due to a long distance transmission when a large amount of data is transmitted over a long distance through a strand of optical fiber. Therefore, since light is directly amplified, the amplification effect is excellent, unlike in a method of converting light into an electrical signal and amplifying the electrical signal, converting the amplified electrical signal into light and transmitting the light. Accordingly, it is economical. An optical signal is amplified by a pumping light. When the pumping light is injected into the erbium doped fiber amplifier, a doped ion such as erbium in the ground state in the optical fiber is excited by the pumping light. The optical signal is amplified by stimulated emission of the excited erbium.
In a wavelength division multiplexed transmission system, various optical signals having different wavelengths are transmitted through an optical fiber. Accordingly, it is possible to efficiently utilize the low loss characteristic of the optical fiber over a broad wavelength band.
As wavelength division multiplexed systems have been developed so that 4 to 16 channels, as well as a single channel, can be transmitted simultaneously, a wavelength division multiplexed erbium doped fiber amplifier (WDM-EDFA) has been developed for this purpose.
In general, in the WDM-EDFA, long distance communications can be performed only when the amplification gain is maintained to be flat in the respective wavelengths with respect to not less than four channels, unlike a single channel. However, even in the gain-flattened WDM-EDFA, a phenomenon may occur in which the gain fails to be flat due to adding or dropping of the respective multiplexed channels or in which the gain of a channel becomes too large or small.
In the methods which have been provided up to now, only the gain flattening of a used wavelength region is considered. However, in these methods, time taken to control the gain according to channel change is not considered in the WDM-EDFA when the multiple channels are added or dropped. Namely, time taken must be considered to obtain a desired output by reading channel information included in a supervision channel, processing the channel information, and controlling the bias current of a pumping light source so as to obtain the most appropriate gain, for the gain flattening of the multi-channels. When the time is not considered, it is useless to control gain since the data channels already pass through the WDM-EDFA before the gain is controlled. Also, though the channel information is sent using a supervision channel, the result is the same if the data channels and the supervision channel are simultaneously transmitted without considering the time taken to control the gain.
To solve the above problem, it is an object of the present invention to provide a wavelength division multiplexed transmission system for solving the instability of amplification according to a difference between amplification gain control time and data channel amplification time in a wavelength division multiplexed erbium doped fiber amplifier (WDM-EDFA) by transmitting the add or drop information of data channels using a supervision channel during an electrical signal processing process before the data channels are multiplexed in an optical terminal station, by delaying the data channels by time for which the supervision channel is delayed on a transmission line, and by transmitting the delayed data channels.
It is another object of the present invention to provide a transmitting method using the same.
Accordingly, to achieve the first object, there is provided a wavelength division multiplexed optical transmission system, comprising a transmitting optical terminal station for transmitting a supervision channel, delaying a plurality of data channels by a predetermined time, and transmitting the delayed data channels, a plurality of wavelength division multiplexed optical amplifying portions for controlling amplification gain according to gain information included in the supervision channel and amplifying the delayed and incident data channels according to the controlled gain, and a receiving optical terminal station for receiving the supervision channel and the data channels output from the wavelength division multiplexed optical amplifying portion.
To achieve the second object, there is provided a method for multiplexing and transmitting a plurality of data channels and a supervision channel using transmitting and receiving optical transmission terminal stations and a wavelength division multiplexed optical amplifying portion positioned on a transmission line between the transmitting and receiving optical terminal stations, comprising the steps of measuring a time difference between the data channels and the supervision channel using a predetermined test signal, constructing a supervision channel by sensing a change in the respective data channels, determining the gain of the wavelength division multiplexed amplifying portion by the transmitting terminal station, and transmitting the supervision channel, controlling the gain of the wavelength division multiplexed amplifying portion according to the amplification gain information included in the supervision channel, delaying the data channels by the time difference by the transmitting optical terminal station and transmitting the delayed data channels to the wavelength division multiplexed optical amplifying portion, and amplifying and transmitting the data channels according to the gain controlled by the wavelength division multiplexed amplifying portion.